In recent years, 2,3,3,3-tetrafluoropropene (which is represented by CF3CF═CH2, hereinafter referred to also as HFO-1234yf) as one of isomers of tetrafluoropropene, has attracted attention as a new refrigerant to replace a chlorofluorocarbon (CFC), a hydrochlorofluorocarbon (HCFC) and a hydrofluorocarbon (HFC) which are greenhouse gases that deplete the ozone layer.
Such HFO-1234yf is stored and transported as filled in a closed container under pressure at ordinary temperature or lower, or as liquefied and filled under pressure under cooling. HFO-1234yf filled in a closed container as such is in a gaseous-liquid state composed of a gas phase and a liquid phase. Further, the HFO-1234yf in a gaseous-liquid state has been required to be stably stored without undergoing reaction such as polymerization, for maintaining quality as a refrigerant or preventing adhesion of impurities (solids) in the container.
It has been known that fluoroolefins undergo polymerization reaction in the presence of oxygen which functions as a radical source. Among fluoroolefins, tetrafluoroethylene is polymerized in the presence of oxygen in a very small amount of from one to a few ten ppm, and in some cases, such a polymerization reaction proceeds explosively. For example, Patent Document 1 discloses that tetrafluoroethylene undergoes polymerization at an oxygen content of 1.4 ppm to form polytetrafluoroethylene. Accordingly, in a case where a fluoroolefin is stored, it is important to reduce oxygen to the utmost limit before treating the fluoroolefin.
However, in order to reduce oxygen to the utmost limit, it is necessary to take new measures such as a step of reducing oxygen to the utmost limit in the production process, and this results in cost increase. Further, if the step of reducing oxygen to the utmost limit is carried out, a yield also tends to be low, and this results in production cost increase.
It remains unknown what extent tetrafluoropropene such as HFO-1234yf is stable to the self-polymerization reaction in the presence of oxygen. In order to maintain the quality as a refrigerant and to carry out storage and transportation safely and stably at a low cost, a method of storing tetrafluoropropene such as HFO-1234yf without occurrence of polymerization reaction has been required.
Heretofore, several proposals for stabilizing hydrofluoropropene have been made. Patent Document 2 proposes a method of adding a stabilizer such as an alkylcatechol or an alkoxyphenol so that hydrofluoropropene is maintained in a stable state (a state where no acid is formed) even in coexistence of air. Further, Patent Document 3 proposes a method of stabilizing hydrofluoropropene by adding a C1-4 aliphatic alcohol as a stabilizer to the hydrofluoropropene.
However, both of the methods of Patent Document 2 and Patent Document 3 are based on the premise of the presence of a refrigerating machine oil, they are a method of stabilizing an entire cooling system by stabilizing a refrigerant composition, and their conditions are different from the stabilization of a refrigerant in a container for storage or transportation, and therefore it is difficult to apply these methods to a method of storing a refrigerant in a container. Further, in the method of adding a stabilizer, it is necessary to remove the stabilizer before use of a refrigerant, and therefore not only loads of a step increases, but also there is a case where the stabilizer is not completely removed in a physical purification method such as distillation, such being undesirable in view of quality control.